This invention relates to a heat insulator unit for puffer type and arc assisted type circuit breakers which utilize gas, such as sulfur hexafluoride (SF.sub.6), as an electrical insulator, interrupting media and mechanical damper in the circuit breakers. Sulfur hexafluoride (SF.sub.6) gas has unique properties that allow it to act as an electrical insulator, interrupting media and mechanical damper in circuit breakers. Those properties include: a high molecular weight; density that is about 41/2 times that of air; excellent thermoconductor; and excellent electrical insulator. One serious drawback in the physical properties of SF.sub.6 gas is that at normal utilization pressures it will liquefy at approximately -30.degree. C. or -22.degree. C. (temperature at which SF.sub.6 gas will liquefy may vary with pressure). The liquification of the SF.sub.6 can reduce the working pressure in the tank below minimum values for interruption or dielectric strength. Thus any circuit breaker utilizing SF.sub.6 gas that is used in the northern tier of the United States, Canada, or elsewhere in cold weather climates around the world must have heaters for the SF.sub.6 circuit breaker tanks.
In states such as North Dakota, Minnesota, Wisconsin and Montana, temperature can drop as low as -50.degree. F. air and -75.degree. F. wind-chill. Such temperatures cause severe operating duties on a Utility's electrical apparatus', in particular the Utility's SF.sub.6 gas based circuit breakers. Although SF.sub.6 gas based circuit breakers are specified to at least -40.degree. C. and in some cases -45.degree. or -50.degree. C., Utility's are experiencing significant problems as a result of the cold weather. These problems include alarms being activated and in some instances circuit breaker lock-outs. An operating Utility is faced with three possible problems when cold weather begins to affect its circuit breakers. The first is the possibility that the sensors activating the alarm are defective. The second problem may be that the circuit breaker tank is low on gas. The third problem may be that the circuit breaker's SF.sub.6 gas has liquefied (or become solid) as a result of the cold temperatures. In addition, a circuit breaker locking out may cause the additional problem of cascading failures. In any of these above listed possible problems, a service worker must be called out to determine the extent of the problem.
In order to operate circuit breakers utilizing SF.sub.6 gas in extreme cold weather conditions, vendors have universally added additional heat to circuit breaker tanks. Venders add heat to the tanks through the use of heaters of various designs. Some of the heater designs are similar to those that are in a conventional electric oven. In this style, the heater is comprised of a heated tube that is enclosed in a configuration that partially surrounds the breaker tank. The heated tube is not in direct contact with the tank and simply heats the air immediately surrounding the heat tube. This type of heater typically uses a bandwidth for temperature control. The use of a bandwidth for the temperature control is necessary to allow for lead time from when the heater is turned on and to store sufficient heat in the tank. Otherwise, excessive cycling would occur. Because these types of heaters are heating the air surrounding the tank, and not the tank itself, they waste a tremendous amount of energy and require a tremendous amount of energy in order to raise the temperature of the tank. Further, these heaters are not insulated. Excessive amounts of electric heat to the circuit breaker tanks places an undue burden on the station service. Solving the circuit breaker tank temperature problems by adding additional heat simply exacerbates the Utility's peak demand problems. Peak demand is the worst time of operation for a utility to be using its own energy to run heaters because this is energy that is needed by customers.
In areas where the temperature drops as low as -50.degree. F. air and -75.degree. F. wind-chill, Utility's use a tremendous amount of energy for the circuit breaker tank heaters. Some vendors use as much as 8000 watts of heat per breaker. Notwithstanding the loss of revenue which could be generated from the sale of the energy used to run the heaters, the use of energy to run heaters is necessary in order to keep the gas within the breaker tanks from liquefying. A circuit breaker tank having a significant amount of SF.sub.6 gas that liquefies renders it unoperational. The circuit breaker is not operational because liquefaction of the SF.sub.6 gas causes the circuit breaker to be locked into an open or closed position by the control instruction.
A utility company may experience significant losses when its circuit breakers are not operational. The costs arise from the need to send qualified technicians to the site to verify status. Loss of customer load may also occur. The need to transfer trip circuit breakers may occur because of the circuit breaker being in a locked close position. This causes the circuit breaker to be non-operational, thereby enlarging the outage and time duration of a fault. There are also expenses resulting from a service call to the circuit breaker if the circuit breaker doesn't reset itself upon the gas pressure rising to an appropriate level.
Heaters that are presently being used on circuit breakers utilizing SF.sub.6 gas are not designed for the effects the wind-chill can have on the exposed tank. There is a need for a heating system that addresses the ambient temperature as well as the rate at which the tank temperature and pressure will drop from extreme wind-chills. There is also a need for a heating system that is more energy efficient.